Quantum news biweekly vol.69, 22nd February — 7th March
TL;DR
- Scientists make progress in understanding microscopic gravity, detecting a weak gravitational pull on a tiny particle.
- Niobium, once considered subpar for superconducting qubits, now serves as the foundation for high-quality engineered qubits.
- A new method utilizing photon detectors enables scientists to characterize optical quantum states, a crucial tool for quantum information processing.
- Josephson tunnel junctions, vital for superconducting quantum computers, are found to be more complex than previously thought, potentially increasing stability.
- Physicists identify a unique fractional quantum Hall state, suggesting the existence of emergent particles known as six-flux composite fermions.
- Quantum phenomena are controlled at room temperature, marking a significant milestone in scientific achievement.
- Simple pentalayer graphene displays the fractional quantum Hall effect, offering potential advancements in the development of robust quantum computers.
- Laser pulses induce a more ordered ferroelectric state in SrTiO3, providing insights into light-induced ferroelectric state development.
- A research team manipulates the chirality of individual molecules and synthesizes highly reactive diradicals using a scanning tunneling microscope probe at low temperatures.
- A novel ligand exchange technique facilitates the synthesis of stable organic cation-based perovskite quantum dots, enhancing solar cell photoactive layer stability.
- And more!
Quantum Computing Market
According to the recent market research report ‘Quantum Computing Market with COVID-19 impact by Offering (Systems and Services), Deployment (On Premises and Cloud Based), Application, Technology, End-use Industry and Region — Global Forecast to 2026’, published by MarketsandMarkets, the Quantum Computing market is expected to grow from USD 472 million in 2021 to USD 1,765 million by 2026, at a CAGR of 30.2%. The early adoption of quantum computing in the banking and finance sector is expected to fuel the growth of the market globally. Other key factors contributing to the growth of the quantum computing market include rising investments by governments of different countries to carry out research and development activities related to quantum computing technology. Several companies are focusing on the adoption of QCaaS post-COVID-19. This, in turn, is expected to contribute to the growth of the quantum computing market. However, stability and error correction issues are expected to restrain the growth of the market.
According to ‘Quantum Computing Market Research Report: By Offering, Deployment Type, Application, Technology, Industry — Industry Share, Growth, Drivers, Trends and Demand Forecast to 2030’ report, the quantum computing market is projected to reach $64,988 million by 2030. Machine learning (ML) is expected to progress at the highest CAGR, during the forecast period, among all application categories, owing to the fact that quantum computing is being integrated in ML for improving the latter’s use case.
Latest Research
Measuring gravity with milligram levitated masses
by Tim M. Fuchs, Dennis G. Uitenbroek, Jaimy Plugge, Noud van Halteren, Jean-Paul van Soest, Andrea Vinante, Hendrik Ulbricht, Tjerk H. Oosterkamp in Science Advances
Scientists are a step closer to unravelling the mysterious forces of the universe after working out how to measure gravity on a microscopic level.
Experts have never fully understood how the force which was discovered by Isaac Newton works in the tiny quantum world. Even Einstein was baffled by quantum gravity and, in his theory of general relativity, said there is no realistic experiment which could show a quantum version of gravity. But now physicists at the University of Southampton, working with scientists in Europe, have successfully detected a weak gravitational pull on a tiny particle using a new technique. They claim it could pave the way to finding the elusive quantum gravity theory.
The experiment used levitating magnets to detect gravity on microscopic particles — small enough to boarder on the quantum realm. Lead author Tim Fuchs, from the University of Southampton, said the results could help experts find the missing puzzle piece in our picture of reality.
He added: “For a century, scientists have tried and failed to understand how gravity and quantum mechanics work together. “Now we have successfully measured gravitational signals at a smallest mass ever recorded, it means we are one step closer to finally realising how it works in tandem.
“From here we will start scaling the source down using this technique until we reach the quantum world on both sides. “By understanding quantum gravity, we could solve some of the mysteries of our universe — like how it began, what happens inside black holes, or uniting all forces into one big theory.”
The rules of the quantum realm are still not fully understood by science — but it is believed that particles and forces at a microscopic scale interact differently than regular-sized objects.
Academics from Southampton conducted the experiment with scientists at Leiden University in the Netherlands and the Institute for Photonics and Nanotechnologies in Italy. Their study used a sophisticated setup involving superconducting devices, known as traps, with magnetic fields, sensitive detectors and advanced vibration isolation. It measured a weak pull, just 30aN, on a tiny particle 0.43mg in size by levitating it in freezing temperatures a hundredth of a degree above absolute zero — about minus-273 degrees Celsius.
The results open the door for future experiments between even smaller objects and forces, said Professor of Physics Hendrik Ulbricht also at the University of Southampton.
He added: “We are pushing the boundaries of science that could lead to new discoveries about gravity and the quantum world. “Our new technique that uses extremely cold temperatures and devices to isolate vibration of the particle will likely prove the way forward for measuring quantum gravity.
“Unravelling these mysteries will help us unlock more secrets about the universe’s very fabric, from the tiniest particles to the grandest cosmic structures.”
Improved coherence in optically defined niobium trilayer-junction qubits
by Alexander Anferov, Kan-Heng Lee, Fang Zhao, Jonathan Simon, David I. Schuster in Physical Review Applied
For years, niobium was considered an underperformer when it came to superconducting qubits. Now scientists supported by Q-NEXT have found a way to engineer a high-performing niobium-based qubit and so take advantage of niobium’s superior qualities.
When it comes to quantum technology, niobium is making a comeback. For the past 15 years, niobium has been sitting on the bench after experiencing a few mediocre at-bats as a core qubit material.
Qubits are the fundamental components of quantum devices. One qubit type relies on superconductivity to process information. Touted for its superior qualities as a superconductor, niobium was always a promising candidate for quantum technologies. But scientists found niobium difficult to engineer as a core qubit component, and so it was relegated to the second string on Team Superconducting Qubit. Now, a group led by Stanford University’s David Schuster has demonstrated a way to create niobium-based qubits that rival the state-of-the-art for their class.
“This was a promising first foray, having resurrected niobium junctions. … With niobium-based qubits’ broad operational reach, we open up a whole new set of capabilities for future quantum technologies.” — David Schuster, Stanford University
“We’ve shown that niobium is relevant again, expanding the possibilities of what we can do with qubits,” said Alexander Anferov of the University of Chicago’s Physical Science division, one of the lead scientists of the result.
By harnessing niobium’s standout features, scientists will be able to expand the capabilities of quantum computers, networks and sensors. These quantum technologies draw on quantum physics to process information in ways that outclass their traditional counterparts and are expected to improve areas as varied as medicine, finance and communication.
When it comes to superconducting qubits, aluminum has ruled the roost. Aluminum-based superconducting qubits can store information for a relatively long time before the data inevitably disintegrates. These longer coherence times mean more time for processing information. The longest coherence times for an aluminum-based superconducting qubit are a few hundred millionths of a second. By contrast, in recent years, the best niobium-based qubits yielded coherence times that are 100 times shorter — a few hundred billionths of a second.
Despite that short qubit lifetime, niobium held attractions. A niobium-based qubit can operate at higher temperatures than its aluminum counterpart and so would require less cooling. It can also operate across an eight-times-greater frequency range and a massive 18,000-times-wider magnetic field range compared to aluminum-based qubits, expanding the menu of uses for the superconducting-qubit family. In one respect, there was no contest between the two materials: Niobium’s operating range trounced aluminum’s. But for years, the short coherence time made the niobium-based qubit a nonstarter.
“No one really made that many qubits out of niobium junctions because they were limited by their coherence,” Anferov said. “But our group wanted to make a qubit that could work at higher temperatures and a greater frequncy range — at 1 K and 100 gigahertz. And for both of those properties, aluminum is not sufficient. We needed something else.”
So, the team had another look at niobium. Specifically, they had a look at the niobium Josephson junction. The Josephson junction is the information-processing heart of the superconducting qubit.
In classical information processing, data comes in bits that are either 0s or 1s. In quantum information processing, a qubit is a mixture of 0 and 1. The superconducting qubit’s information “lives” as a mixture of 0 and 1 inside the junction. The longer the junction can sustain the information in that mixed state, the better the junction and the better the qubit.
The Josephson junction is structured like a sandwich, consisting of a layer of nonconducting material squeezed between two layers of superconducting metal. A conductor is a material that provides easy passage for electrical current. A superconductor kicks it up a notch: It carries electrical current with zero resistance. Electromagnetic energy flows between the junction’s outer layers in the mixed quantum state.
The typical, trusty aluminum Josephson junction is made of two layers of aluminum and a middle layer of aluminum oxide. A typical niobium junction is made of two layers of niobium and a middle layer of niobium oxide.
Schuster’s group found that the junction’s niobium oxide layer sapped the energy required to sustain quantum states. They also identified the niobium junctions’ supporting architecture as a big source of energy loss, causing the qubit’s quantum state to fizzle out. The team’s breakthrough involved both a new junction arrangement and a new fabrication technique.
The new arrangement called on a familiar friend: aluminum. The design did away with the energy-sucking niobium oxide. And instead of two distinct materials, it used three. The result was a low-loss, trilayer junction — niobium, aluminum, aluminum oxide, aluminum, niobium.
“We did this best-of-both-worlds approach,” Anferov said. “The thin layer of aluminum can inherit the superconducting properties of the niobium nearby. This way, we can use the proven chemical properties of aluminum and still have the superconducting properties of niobium.”
The group’s fabrication technique involved removing scaffolding that supported the niobium junction in previous schemes. They found a way to maintain the junction’s structure while getting rid of the loss-inducing, extraneous material that hampered coherence in previous designs.
“It turns out just getting rid of the garbage helped,” Anferov said.
After incorporating their new junction into superconducting qubits, the Schuster group achieved a coherence time of 62 millionths of a second, 150 times longer than its best-performing niobium predecessors. The qubits also exhibited a quality factor — an index of how well a qubit stores energy — of 2.57 x 105, a 100-fold improvement over previous niobium-based qubits and competitive with aluminum-based qubit quality factors.
“We’ve made this junction that still has the nice properties of niobium, and we’ve improved the loss properties of the junction,” Anferov said. “We can directly outperform any aluminum qubit because aluminum is an inferior material in many ways. I now have a qubit that doesn’t die at higher temperatures, which is the big kicker.”
Low-noise balanced homodyne detection with superconducting nanowire single-photon detectors
by Maximilian Protte, Timon Schapeler, Jan Sperling, Tim J. Bartley in Optica Quantum
Scientists at Paderborn University have used a new method to determine the characteristics of optical, i.e. light-based, quantum states. For the first time, they are using certain photon detectors — devices that can detect individual light particles — for so-called homodyne detection. The ability to characterise optical quantum states makes the method an essential tool for quantum information processing. Precise knowledge of the characteristics is important for use in quantum computers, for example.
“Homodyne detection is a method frequently used in quantum optics to investigate the wave-like nature of optical quantum states,” explains Timon Schapeler from the Paderborn “Mesoscopic Quantum Optics” working group at the Department of Physics. Together with Dr Maximilian Protte, he has used the method to investigate the so-called continuous variables of optical quantum states. This involves the variable properties of light waves. These can be, for example, the amplitude or phase, i.e. the oscillation behaviour of waves, which are important for the targeted manipulation of light, among other things.
For the first time, the physicists have used superconducting nanowire single photon detectors for the measurements — currently the fastest devices for photon counting. With their special experimental setup, the two scientists have shown that a homodyne detector with superconducting single photon detectors has a linear response to the input photon flux. Translated, this means that the measured signal is proportional to the input signal.
“In principle, the integration of superconducting single-photon detectors brings many advantages in the area of continuous variables, not least the intrinsic phase stability. These systems also have almost 100 per cent on-chip detection efficiency. This means that no particles are lost during detection. Our results could enable the development of highly efficient homodyne detectors with single-photon sensitive detectors,” says Schapeler.
Working with continuous variables of light opens up new and exciting possibilities in quantum information processing beyond qubits, the usual computing units of quantum computers.
Observation of Josephson harmonics in tunnel junctions
by Dennis Willsch, Dennis Rieger, Patrick Winkel, et al in Nature Physics
Quantum bits can be described more precisely with the help of newly discovered harmonics as a team of 30 researchers reports.
Physicists from Forschungszentrum Jülich and the Karlsruhe Institute of Technology have uncovered that Josephson tunnel junctions — the fundamental building blocks of superconducting quantum computers — are more complex than previously thought. Just like overtones in a musical instrument, harmonics are superimposed on the fundamental mode. As a consequence, corrections may lead to quantum bits that are 2 to 7 times more stable. The researchers support their findings with experimental evidence from multiple laboratories across the globe, including the University of Cologne, Ecole Normale Supérieure in Paris, and IBM Quantum in New York.
It all started in 2019, when Dennis Willsch and Dennis Rieger — two PhD students from FZJ and KIT at the time and joint first authors of the paper — were having a hard time understanding their experiments using the standard model for Josephson tunnel junctions. This model had won Brian Josephson the Nobel Prize in Physics in 1973. Excited to get to the bottom of this, the team led by Ioan Pop scrutinized further data from the Ecole Normale Supérieure in Paris and a 27-qubit device at IBM Quantum in New York, as well as data from previously published experiments. Independently, researchers from the University of Cologne were observing similar deviations of their data from the standard model.
“Fortunately, Gianluigi Catelani, who was involved in both projects and realized the overlap, brought the research teams together!,” recalls Dennis Willsch from FZ Jülich. “The timing was perfect,” adds Chris Dickel from the University of Cologne, “since, at that time, we were exploring quite different consequences of the same underlying problem.”
Josephson tunnel junctions consist of two superconductors with a thin insulating barrier in-between and, for decades, these circuit elements have been described with a simple sinusoidal model. However, as the researchers demonstrate, this “standard model” fails to fully describe the Josephson junctions that are used to build quantum bits. Instead, an extended model including higher harmonics is required to describe the tunneling current between the two superconductors. The principle can also be found in the field of music. When the string of an instrument is struck, the fundamental frequency is overlaid by several harmonic overtones.
“It’s exciting that the measurements in the community have reached the level of accuracy at which we can resolve these small corrections to a model which has been considered sufficient for more than 15 years,” Dennis Rieger remarks.
When the four coordinating professors — Ioan Pop from KIT and Gianluigi Catelani, Kristel Michielsen and David DiVincenzo from FZJ — realized the impact of the findings, they brought together the large collaboration of experimentalists, theoreticians, and material scientists, to join their efforts in presenting a compelling case for the Josephson harmonics model. In the publication, the researchers explore the origin and consequences of Josephson harmonics. “As an immediate consequence, we believe that Josephson harmonics will help in engineering better and more reliable quantum bits by reducing errors up to an order of magnitude, which brings us one step closer towards the dream of a fully universal superconducting quantum computer,” the two first authors conclude.
Evidence for Topological Protection Derived from Six-Flux Composite Fermions
by Haoyun Huang, Waseem Hussain, S. A. Myers, L. N. Pfeiffer, K. W. West, K. W. Baldwin, G. A. Csáthy in Nature Communications
If the fractional quantum Hall regime were a series of highways, these highways would have either two or four lanes. The flow of the two-flux or four-flux composite fermions, like automobiles in this two- to four-flux composite fermion traffic scenario, naturally explain the more than 90 fractional quantum Hall states that form in a large variety of host materials. Physicists at Purdue University have recently discovered, though, that fractional quantum Hall regimes are not limited to two-flux or four-flux and have discovered the existence of a new type of emergent particle, which they are calling six-flux composite fermion.
Gabor Csathy, professor and head of the Department of Physics and Astronomy at the Purdue University College of Science, along with PhD students Haoyun Huang, Waseem Hussain, and recent PhD graduate Sean Myers, led this discovery from the West Lafayette campus of Purdue. Csathy credits lead author Huang as having conceived, led the measurements and writing a large part of the manuscript. All the ultra-low-temperature measurements were completed in Csathy’s Physics Building lab. In his lab they conduct research on strongly correlated electron physics, sometimes referred to as topological electron physics.
Weak interactions of electrons are well established, and the behavior is quite predictable. When electrons interact weakly, the electron is commonly considered the natural building block of the entire system. But when the electrons interact strongly, interpreting the systemic behavior by thinking of individual electrons becomes nearly impossible.
“This occurs in very few instances, like in the fractional quantum Hall regime which we study, for example,” says Csathy. “To explain fractional quantum Hall states, the composite fermion, a very intuitive fundamental building block, comes in different flavors. They can account for a whole subset of the fractional quantum Hall states. But all the fully developed, (i.e topologically protected), fractional quantum Hall states could be accounted for by only two types of composite fermions: the two-flux and four-flux composite fermions. Here we reported a new fractional quantum Hall state that cannot be explained by any of these previous ideas! Instead, we need to invoke the existence of a new type of emergent particle, the so-called six-flux composite fermions. The discovery of new fractional quantum Hall states is scarce enough. However, the discovery of a new emergent particle in condensed matter physics is truly rare and amazing.”
For now, these ideas will be used to expand our understanding of the ordering of the known fractional quantum Hall states into a “periodic table.” It is especially notable to this process that the emergent composite fermion particle is unique in that the electron captures six quantized magnetic flux quanta, forming the most intricate composite fermion known to date.
“The numerology of this complicated physics puzzle requires quite some patience,” says Haoyun Huang, Csathy’s PhD student. “Take the nu=2/3 fractional state as an example. Since 2/3=2/(2*2–1), the nu=2/3 state belongs to the two-flux family. Similarly, for the nu=2/7 fractional state, 2/7=2/(2*4–1), so this state belongs to the four-flux family. In contrast, the fractional states we discovered closely relate to 2/11=2/(2*6–1). Before our work, no fully quantized fractional quantum Hall state was seen that could be associated with six-flux composite fermions. The situation was completely different on the theory front: The existence of these kinds of composite fermions was predicted by Jainendra Jain in his highly influential theory of composite fermions published in 1989. The associated quantization was not observed during these 34 years.”
The material used in this study was grown by a Princeton University team led by Loren Pfeiffer. The GaAs semiconductor electrical quality played a huge role in the success of this research. According to Csathy, this Princeton group is leading the world in growing the highest quality GaAs-based materials.
“The GaAs they grow is very special, as the number of imperfections is astonishingly low,” he says. “The combination of low disorder and the ultra-low-temperature measurement expertise in the Csathy lab made this project possible. One reason we were measuring these samples is that very recently the Princeton group has significantly improved the quality of the GaAs semiconductor, as measured by the tiny amounts of defects present. These improved samples will, for sure, continue to constitute a playground for new physics.”
This exciting discovery is part of ongoing research by Csathy’s team. The team continues to push the limits of discovery in their persistent pursuit of topological electron physics.
Room-temperature quantum optomechanics using an ultralow noise cavity
by Guanhao Huang, Alberto Beccari, Nils J. Engelsen, Tobias J. Kippenberg in Nature
In the realm of quantum mechanics, the ability to observe and control quantum phenomena at room temperature has long been elusive, especially on a large or “macroscopic” scale. Traditionally, such observations have been confined to environments near absolute zero, where quantum effects are easier to detect. But the requirement for extreme cold has been a major hurdle, limiting practical applications of quantum technologies.
Now, a study led by Tobias J. Kippenberg and Nils Johan Engelsen at EPFL, redefines the boundaries of what’s possible. The pioneering work blends quantum physics and mechanical engineering to achieve control of quantum phenomena at room temperature.
“Reaching the regime of room temperature quantum optomechanics has been an open challenge since decades,” says Kippenberg. “Our work realizes effectively the Heisenberg microscope — long thought to be only a theoretical toy model.”
In their experimental setup, the researchers created an ultra-low noise optomechanical system — a setup where light and mechanical motion interconnect, allowing them to study and manipulate how light influences moving objects with high precision.
The main problem with room temperature is thermal noise, which perturbs delicate quantum dynamics. To minimize that, the scientists used cavity mirrors, which are specialized mirrors that bounce light back and forth inside a confined space (the cavity), effectively “trapping” it and enhancing its interaction with the mechanical elements in the system. To reduce the thermal noise, the mirrors are patterned with crystal-like periodic (“phononic crystal”) structures.
Another crucial component was a 4mm drum-like device called a mechanical oscillator, which interacts with light inside the cavity. Its relatively large size and design are key to isolating it from environmental noise, making it possible to detect subtle quantum phenomena at room temperature. “The drum we use in this experiment is the culmination of many years of effort to create mechanical oscillators that are well-isolated from the environment,” says Engelsen.
“The techniques we used to deal with notorious and complex noise sources are of high relevance and impact to the broader community of precision sensing and measurement,” says Guanhao Huang, one of the two PhD students leading the project.
The setup allowed the researchers to achieve “optical squeezing,” a quantum phenomenon where certain properties of light, like its intensity or phase, are manipulated to reduce the fluctuations in one variable at the expense of increasing fluctuations in the other, as dictated by Heisenberg’s principle. By demonstrating optical squeezing at room temperature in their system, the researchers showed that they could effectively control and observe quantum phenomena in a macroscopic system without the need for extremely low temperatures. The team believes the ability to operate the system at room temperature will expand access to quantum optomechanical systems, which are established testbeds for quantum measurement and quantum mechanics at macroscopic scales.
“The system we developed might facilitate new hybrid quantum systems where the mechanical drum strongly interacts with different objects, such as trapped clouds of atoms,” adds Alberto Beccari, the other PhD student leading the study. “These systems are useful for quantum information, and help us understand how to create large, complex quantum states.”
Fractional quantum anomalous Hall effect in multilayer graphene
by Zhengguang Lu, Tonghang Han, Yuxuan Yao, Aidan P. Reddy, Jixiang Yang, Junseok Seo, Kenji Watanabe, Takashi Taniguchi, Liang Fu, Long Ju in Nature
The electron is the basic unit of electricity, as it carries a single negative charge. This is what we’re taught in high school physics, and it is overwhelmingly the case in most materials in nature. But in very special states of matter, electrons can splinter into fractions of their whole. This phenomenon, known as “fractional charge,” is exceedingly rare, and if it can be corralled and controlled, the exotic electronic state could help to build resilient, fault-tolerant quantum computers.
To date, this effect, known to physicists as the “fractional quantum Hall effect,” has been observed a handful of times, and mostly under very high, carefully maintained magnetic fields. Only recently have scientists seen the effect in a material that did not require such powerful magnetic manipulation. Now, MIT physicists have observed the elusive fractional charge effect, this time in a simpler material: five layers of graphene — an atom-thin layer of carbon that stems from graphite and common pencil lead.
They found that when five sheets of graphene are stacked like steps on a staircase, the resulting structure inherently provides just the right conditions for electrons to pass through as fractions of their total charge, with no need for any external magnetic field. The results are the first evidence of the “fractional quantum anomalous Hall effect” (the term “anomalous” refers to the absence of a magnetic field) in crystalline graphene, a material that physicists did not expect to exhibit this effect.
“This five-layer graphene is a material system where many good surprises happen,” says study author Long Ju, assistant professor of physics at MIT. “Fractional charge is just so exotic, and now we can realize this effect with a much simpler system and without a magnetic field. That in itself is important for fundamental physics. And it could enable the possibility for a type of quantum computing that is more robust against perturbation.”
The fractional quantum Hall effect is an example of the weird phenomena that can arise when particles shift from behaving as individual units to acting together as a whole. This collective “correlated” behavior emerges in special states, for instance when electrons are slowed from their normally frenetic pace to a crawl that enables the particles to sense each other and interact. These interactions can produce rare electronic states, such as the seemingly unorthodox splitting of an electron’s charge.
In 1982, scientists discovered the fractional quantum Hall effect in heterostructures of gallium arsenide, where a gas of electrons confined in a two-dimensional plane is placed under high magnetic fields. The discovery later won the group a Nobel Prize in Physics.
“[The discovery] was a very big deal, because these unit charges interacting in a way to give something like fractional charge was very, very bizarre,” Ju says. “At the time, there were no theory predictions, and the experiments surprised everyone.”
Those researchers achieved their groundbreaking results using magnetic fields to slow down the material’s electrons enough for them to interact. The fields they worked with were about 10 times stronger than what typically powers an MRI machine.
In August 2023, scientists at the University of Washington reported the first evidence of fractional charge without a magnetic field. They observed this “anomalous” version of the effect, in a twisted semiconductor called molybdenum ditelluride. The group prepared the material in a specific configuration, which theorists predicted would give the material an inherent magnetic field, enough to encourage electrons to fractionalize without any external magnetic control.
The “no magnets” result opened a promising route to topological quantum computing — a more secure form of quantum computing, in which the added ingredient of topology (a property that remains unchanged in the face of weak deformation or disturbance) gives a qubit added protection when carrying out a computation. This computation scheme is based on a combination of fractional quantum Hall effect and a superconductor. It used to be almost impossible to realize: One needs a strong magnetic field to get fractional charge, while the same magnetic field will usually kill the superconductor. In this case the fractional charges would serve as a qubit (the basic unit of a quantum computer).
That same month, Ju and his team happened to also observe signs of anomalous fractional charge in graphene — a material for which there had been no predictions for exhibiting such an effect. Ju’s group has been exploring electronic behavior in graphene, which by itself has exhibited exceptional properties. Most recently, Ju’s group has looked into pentalayer graphene — a structure of five graphene sheets, each stacked slightly off from the other, like steps on a staircase. Such pentalayer graphene structure is embedded in graphite and can be obtained by exfoliation using Scotch tape. When placed in a refrigerator at ultracold temperatures, the structure’s electrons slow to a crawl and interact in ways they normally wouldn’t when whizzing around at higher temperatures.
In their new work, the researchers did some calculations and found that electrons might interact with each other even more strongly if the pentalayer structure were aligned with hexagonal boron nitride (hBN) — a material that has a similar atomic structure to that of graphene, but with slightly different dimensions. In combination, the two materials should produce a moiré superlattice — an intricate, scaffold-like atomic structure that could slow electrons down in ways that mimic a magnetic field.
“We did these calculations, then thought, let’s go for it,” says Ju, who happened to install a new dilution refrigerator in his MIT lab last summer, which the team planned to use to cool materials down to ultralow temperatures, to study exotic electronic behavior.
The researchers fabricated two samples of the hybrid graphene structure by first exfoliating graphene layers from a block of graphite, then using optical tools to identify five-layered flakes in the steplike configuration. They then stamped the graphene flake onto an hBN flake and placed a second hBN flake over the graphene structure. Finally, they attached electrodes to the structure and placed it in the refrigerator, set to near absolute zero. As they applied a current to the material and measured the voltage output, they started to see signatures of fractional charge, where the voltage equals the current multiplied by a fractional number and some fundamental physics constants.
“The day we saw it, we didn’t recognize it at first,” says first author Lu. “Then we started to shout as we realized, this was really big. It was a completely surprising moment.”
“This was probably the first serious samples we put in the new fridge,” adds co-first author Han. “Once we calmed down, we looked in detail to make sure that what we were seeing was real.”
With further analysis, the team confirmed that the graphene structure indeed exhibited the fractional quantum anomalous Hall effect. It is the first time the effect has been seen in graphene.
“Graphene can also be a superconductor,” Ju says. “So, you could have two totally different effects in the same material, right next to each other. If you use graphene to talk to graphene, it avoids a lot of unwanted effects when bridging graphene with other materials.”
For now, the group is continuing to explore multilayer graphene for other rare electronic states.
“We are diving in to explore many fundamental physics ideas and applications,” he says. “We know there will be more to come.”
Quenched lattice fluctuations in optically driven SrTiO3
by M. Fechner, M. Först, G. Orenstein, V. Krapivin, A. S. Disa, M. Buzzi, A. von Hoegen, G. de la Pena, Q. L. Nguyen, R. Mankowsky, M. Sander, H. Lemke, Y. Deng, M. Trigo, A. Cavalleri in Nature Materials
Mid-infrared and terahertz frequency laser pulses are powerful tools to manipulate the properties of quantum materials through tailored modifications of their crystal structure. Light-induced ferroelectricity in SrTiO3 is a remarkable demonstration of these physics. Under mid-infrared illumination, this material transforms into a state of permanently ordered electrical dipoles, which is absent in its equilibrium phase diagram. The mechanism underlying this transformation is not understood.
Now, a team of researchers of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Germany and the SLAC National Accelerator Laboratory in the United States has performed an experiment at the SwissFEL X-ray Free-Electron Laser to identify the intrinsic interactions relevant to creating this state. The new insight was gained not by detecting the position of the atoms, but by measuring the fluctuations of these atomic positions. The result provides evidence that these fluctuations are reduced, which may explain why the dipolar structure is more ordered than in equilibrium, and why a ferroelectric state could be induced.
Ferroelectric materials are characterized by the spontaneous parallel alignment of electric dipoles, leading to a macroscopic polarization that can point in two opposite directions. The pointing direction can be switched by an electric field, enabling the use of ferroelectrics in the digital storage and processing components of modern electronic devices.
Strontium titanate, SrTiO3, is a so-called quantum paraelectric. Unlike many of the ferroelectric materials, SrTiO3 lacks a macroscopic ferroelectric state. Yet, abundant experimental evidence shows that quantum fluctuations of the crystal lattice prevent the long-range order from developing. Surprisingly, in 2019 the Cavalleri group found that SrTiO3 transforms into a ferroelectric when certain vibrations of the crystal lattice are excited by intense pulses in the mid-infrared. The use of light to induce and control ferroelectricity at electronically inaccessible high frequencies can be envisioned as the key element of future high-speed memory applications.
At the time, the nonlinear response of the crystal lattice was speculated to be the origin of this effect, resulting in the formation of strain that helps the material to become ferroelectric. However, direct measurements of the strain and, even more importantly, of the fluctuations of the atomic positions on the earliest timescales after the mid-IR excitation were lacking. The researchers teamed up with Mariano Trigo’s group at SLAC and combined the mid-infrared excitation with femtosecond X-ray pulses from the SwissFEL free electron laser to shine light on these dynamics, which take place on the sub-picosecond time scale — shorter than a trillionth of a second.
“In a typical X-ray diffraction experiment, one makes use of the constructive interference of the X-rays scattered from the periodically aligned atoms to measure their average positions,” says Michael Först, one of the leading authors of this work. “But here, we detected the diffuse scattering arising from disorder in the atomic arrangement which is sensitive to fluctuations, in other words noise, of the crystal lattice.”
Experimentally, the team found that the fluctuations of certain rotational modes in the SrTiO3 lattice, which obstruct the formation of long-range ferroelectricity, were rapidly reduced by the pulsed mid-infrared excitation. Such suppression does not occur in this material in equilibrium and hints at the origin of the light-induced ferroelectricity. This was confirmed by a rigorous theoretical analysis that revealed complex, high-order interactions between a set of lattice vibrations and the strain as the source of these observations. Michael Fechner, the theorist of this project, emphasizes the importance of the collaboration between theory and experiment: “It allows us to sharpen our tools for predictions and, consequently, to enhance our understanding of matter and its interactions with light.”
Andrea Cavalleri, group leader and director at the MPSD, foresees new opportunities arising from this study: “The fact that certain lattice fluctuations, which prevent the formation of long-range ferroic order, can be suppressed by dynamic means is new and offers possibilities for similar behavior in other quantum materials. Furthermore, as our group studies induced order in other settings, including magnetic and superconducting, the results discussed here may have wider implications beyond the physics of SrTiO3.”
Local probe-induced structural isomerization in a one-dimensional molecular array
by Shigeki Kawai, Orlando J. Silveira, Lauri Kurki, Zhangyu Yuan, Tomohiko Nishiuchi, Takuya Kodama, Kewei Sun, Oscar Custance, Jose L. Lado, Takashi Kubo, Adam S. Foster in Nature Communications
An international research team led by NIMS, the Osaka University Graduate School of Science and the Kanazawa University Nano Life Science Institute (WPI-NanoLSI) has succeeded for the first time in controlling the chirality of individual molecules through structural isomerization. The team also succeeded in synthesizing highly reactive diradicals with two unpaired electrons. These achievements were made using a scanning tunneling microscope probe at low temperatures.
It is usually quite challenging to control the chirality of individual molecular units and synthesize extremely reactive diradicals in organic chemistry, preventing detailed investigation of the electronic and magnetic properties of diradicals. These issues had inspired the development of chemical reaction techniques to control structures of individual molecules on surface.
This research team recently developed a technique, which allows them to modify the chirality of specific individual molecular units in a three-dimensional nanostructure in a controlled manner. This was achieved by exciting a target molecular unit with tunneling current from a scanning tunneling microscope probe at low temperature under ultrahigh vacuum conditions. By precisely controlling current injection parameters (e.g., the molecular site, at which the tunneling current is injected at a given applied voltage), the team was able to rearrange molecular units into three different configurations: two different stereoisomers and a diradical. Finally, the team demonstrated the controllability and reproducibility of the structural isomerization by encoding ASCII characters using binary and ternary values in a series of one-dimensional molecular arrays with each array representing a single character.
In future research, the team plans to fabricate novel carbon nanostructures composed of designer molecular units, whose configurations are controlled via the structural isomerization technique developed in this project. In addition, the team will explore the possibility of creating quantum materials, in which radical molecular units lead magnetic exchange couplings between the units as designed — a quantum mechanical effect.
Alkyl ammonium iodide-based ligand exchange strategy for high-efficiency organic-cation perovskite quantum dot solar cells
by Havid Aqoma, Sang-Hak Lee, Imil Fadli Imran, Jin-Ha Hwang, Su-Ho Lee, Sung-Yeon Jang in Nature Energy
A groundbreaking research breakthrough in solar energy has propelled the development of the world’s most efficient quantum dot (QD) solar cell, marking a significant leap towards the commercialization of next-generation solar cells. This cutting-edge QD solution and device have demonstrated exceptional performance, retaining their efficiency even after long-term storage. Led by Professor Sung-Yeon Jang from the School of Energy and Chemical Engineering at UNIST, a team of researchers has unveiled a novel ligand exchange technique. This innovative approach enables the synthesis of organic cation-based perovskite quantum dots (PQDs), ensuring exceptional stability while suppressing internal defects in the photoactive layer of solar cells.
“Our developed technology has achieved an impressive 18.1% efficiency in QD solar cells,” stated Professor Jang. “This remarkable achievement represents the highest efficiency among quantum dot solar cells recognized by the National Renewable Energy Laboratory (NREL) in the United States.”
The increasing interest in related fields is evident, as last year, three scientists who discovered and developed QDs, as advanced nanotechnology products, were awarded the Nobel Prize in Chemistry. QDs are semiconducting nanocrystals with typical dimensions ranging from several to tens of nanometers, capable of controlling photoelectric properties based on their particle size. PQDs, in particular, have garnered significant attention from researchers due to their outstanding photoelectric properties. Furthermore, their manufacturing process involves simple spraying or application to a solvent, eliminating the need for the growth process on substrates. This streamlined approach allows for high-quality production in various manufacturing environments.
However, the practical use of QDs as solar cells necessitates a technology that reduces the distance between QDs through ligand exchange, a process that binds a large molecule, such as a ligand receptor, to the surface of a QD. Organic PQDs face notable challenges, including defects in their crystals and surfaces during the substitution process. As a result, inorganic PQDs with limited efficiency of up to 16% have been predominantly utilized as materials for solar cells.
In this study, the research team employed an alkyl ammonium iodide-based ligand exchange strategy, effectively substituting ligands for organic PQDs with excellent solar utilization. This breakthrough enables the creation of a photoactive layer of QDs for solar cells with high substitution efficiency and controlled defects.
Consequently, the efficiency of organic PQDs, previously limited to 13% using existing ligand substitution technology, has been significantly improved to 18.1%. Moreover, these solar cells demonstrate exceptional stability, maintaining their performance even after long-term storage for over two years. The newly-developed organic PQD solar cells exhibit both high efficiency and stability simultaneously.
“Previous research on QD solar cells predominantly employed inorganic PQDs,” remarked Sang-Hak Lee, the first author of the study. “Through this study, we have demonstrated the potential by addressing the challenges associated with organic PQDs, which have proven difficult to utilize.”
“This study presents a new direction for the ligand exchange method in organic PQDs, serving as a catalyst to revolutionize the field of QD solar cell material research in the future,” commented Professor Jang.
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